These fruit have been produced from 2 different Swingle rootstock seedlings.
They are +tennis ball size this season, but previously larger, closer to grapefruit size. Another tree produces smaller round fruit,( not pictured) but nearly identical to the round fruit in the pic.
All are pretty much the same in flavor, something like grapefruit + a hint of Trifoliata, and a pineapple type scent. The juice is ok, not up there with common Citrus juice,

http://img440.imageshack.us/i/sw1821.jpg/
http://img440.imageshack.us/i/sw1821.jpg/
http://img15.imageshack.us/i/sw1826.jpg/
http://img15.imageshack.us/i/sw1826.jpg/

Very interesting, thanks for making this thread. So you can eat these Swingle fruits fresh off the tree and enjoy them?

No I don't think the average person could eat the fruit from the tree and enjoy, perhaps the people who enjoy wild fruit, eccentrics and die hard home wine makers would though. They seem more acceptable than most citranges, but i haven't eaten a whole one. The juice is interesting. It is passable Citrus juice. A forum member from Germany(I think) posted about his Swingle seedling fruit, saying they were pretty bad tasting, so perhaps climate is influential in fruit flavor, or seedlings are more variable than these examples.
I would probably guess it is climate.